Choice A is the correct answer. Model the decay pattern.

1. Retention: 90% remains means factor $b = 0.9$.
2. Time Period: Every 10 minutes, so exponent is $\frac{t}{10}$.
3. Equation: $A(t) = 500(0.9)^{t/10}$.
4. Verify: At $t=10$, $A = 500(0.9)^1 = 450$ (90% of 500) ✓

💡 Strategic Tip: 90% remaining = factor of 0.9, not 0.1.

Choice B is incorrect because 0.1 would mean only 10% remains. Choice C is incorrect because exponent should be $t/10$, not $10t$. Choice D is incorrect because initial value is 500, and decay happens per 10 min.